Dual hysteresis signal detection circuit

ABSTRACT

A dual hysteresis signal detection circuit is proposed for detecting a duration of a signal with two hysteresis comparators. There is a first band pass filter for inputting the signal and outputting the signal having a first frequency. A first hysteresis comparator electrically couples to the first band pass filter for comparing with the first frequency of the signal to output a first square wave signal according a first high voltage and a first low voltage. A second hysteresis comparator electrically couples to the first band pass filter for comparing with the first frequency of the signal to output a second square wave signal according to a second high voltage and a second low voltage. A first detection circuit electrically couples to the first hysteresis comparator for detecting a part of the signal having the first frequency that is higher than the first high voltage and lower than the first low voltage to output a first detection signal according to the first square wave signal. A second detection circuit electrically couples to the second hysteresis comparator for detecting a part of the signal having the first frequency that is higher than the second high voltage and lower than the second low voltage to output a second detection signal according to the second square wave signal. The duration of the signal being detected according to the first detection signal and the second detection signal.

FIELD OF THE INVENTION

[0001] The present invention is related to a circuit for detecting dualhysteresis signals, and more particularly to the circuit for detecting ainput signal duration with a upper hysteresis comparator and a lowerhysteresis comparator.

BACKGROUND OF THE INVENTION

[0002] In general, when the telephone with the spot dial function isused while the third people calls this telephone, the telephone companywill send a multi-frequency signal to tell the user the spot dialcoming. This is the so-called CAS( CPE( Customer Premise Equipment)Alerting Signal) tone. When the telephone circuit detects this signal,an acknowledged signal will send to the telephone company. The telephonecompany may send the telephone number of the third people and relativemessages to the user telephone. The CAS tone comprises themulti-frequency signal of 2130 HZ and 2750 HZ. When the caller-IDcircuit receives the CAS tone, both the signal frequency and the signallength will be detected. If the frequency and length of the signal meetthe specification, the signal may be acknowledged as the signal sentfrom the telephone company. The Bellcore specification is listed intable 1.

[0003] The conventional detection circuit has the damping effect whenthe CAS tone having large power is received. The effect causes the CAStone received is longer than the real CAS tone transmitted, so the CAStone detection may be incorrect.

[0004] For instance of the CAS tone with high frequency 2750 HZ, whenthe 2750 HZ signal of the input signal is filtered through a band passfilter, the level of “1” or “0” will be decided by a comparator, andsuch a procedure is the so-called analog to digital conversion. Thecomparator output is dependent on the hysteresis. The principle issimilar to the schmitt trigger. FIG. 1 shows the characteristic curve,wherein Verf is the reference voltage in the analog circuit and thevoltage potential from Vth− to Vth+ is the hysteresis value. When the Viis increased, if Vi>Vth+, Vo=“1”, and when the Vi is decreased, ifVi<Vth−, Vo=“0”. The hysteresis value is according to the minimum valueacceptable in the telecommunication specification. The signal will besent to the detection circuit through the hysteresis comparator. Thedetection circuit is consist of counters used to detect the valid inputsignal according to the signal frequency. If the input signal is valid,the detection circuit outputs CAS_ALGO=“1”, on the contrary the CASALGO=“0”.

[0005]FIG. 2 Shows the conventional circuit. A short damping effect willappear in case of a large input signal through the band pass filter 21or 22 after the input signal is ended, but this wrong input signalthrough the detection of hysteresis comparator 23 or 24 may be as avalid signal. Therefore, the valid signal detected will be longer thanthe real signal. The larger the signal strength is, the longer the timeis taken. In speaking of the filter, the calculation time of the digitalfilter is longer than the analog filter, so the damping effect will moreserious with digital filter.

SUMMARY OF THE INVENTION

[0006] An object of the present invention is to provide two hysteresiscomparator for reducing the damping effect.

[0007] Another object of the present invention is to make CAS tonereceived corresponding with the real signal length.

[0008] According to the present invention, a circuit is proposed fordetecting a duration of a signal with two hysteresis comparators. Thereis a first band pass filter for inputting the signal and outputting thesignal having a first frequency. A first hysteresis comparatorelectrically couples to the first band pass filter for comparing withthe first frequency of the signal to output a first square wave signalaccording a first high voltage and a first low voltage. A secondhysteresis comparator electrically couples to the first band pass filterfor comparing with the first frequency of the signal to output a secondsquare wave signal according to a second high voltage and a second lowvoltage. A first detection circuit electrically couples to the firsthysteresis comparator for detecting a part of the signal having thefirst frequency that is higher than the first high voltage and lowerthan the first low voltage to output a first detection signal accordingto the first square wave signal. A second detection circuit electricallycouples to the second hysteresis comparator for detecting a part of thesignal having the first frequency that is higher than the second highvoltage and lower than the second low voltage to output a seconddetection signal according to the second square wave signal. Theduration of the signal being detected according to the first detectionsignal and the second detection signal.

[0009] In accordance with one aspect of the present invention, thesignal is a CAS tone.

[0010] In accordance with one aspect of the present invention, the firstfrequency is 2130 HZ.

[0011] In accordance with one aspect of the present invention, the firsthigh voltage is higher than the second high voltage.

[0012] In accordance with one aspect of the present invention, the firstlow voltage is lower than the second low voltage.

[0013] In accordance with one aspect of the present invention, thesecond square wave signal appears earlier than the first square wavesignal according to the signal.

[0014] In accordance with one aspect of the present invention, thesecond square wave signal leads the square wave signal.

[0015] In accordance with one aspect of the present invention, thecircuit further comprises a second band pass filter for inputting thesignal and outputting the signal having a second frequency; a thirdhysteresis comparator electrically coupled to the second band passfilter for comparing with second frequency of the signal to output athird square wave signal according to the first high voltage and thefirst low voltave; and a fourth hysteresis comparator electricallycoupled to the second band pass filter for comparing with secondfrequency of the signal to output a fourth square wave signal accordingto the second high voltage and the second low voltage.

[0016] In accordance with one aspect of the present invention, thesecond frequency is 2750 HZ.

[0017] In accordance with one aspect of the present invention, thecircuit further comprises a multiplexor electrically coupled to thefirst detection circuit and the second detection circuit for detectingthe duration of the signal according to the first detection signal andthe second detection signal.

[0018] According to the present invention, a circuit is proposed fordetecting a duration of a signal with two hysteresis comparators. Thereis a band pass filter for handling the signal having a first frequencyto output the signal having the first frequency. Two hysteresiscomparators electrically couple to the band pass filter for handling thesignal having the first frequency and forming a first square wave signaland a second square wave signal. Two detection circuits electricallycouple to two hysteresis comparators for forming a first detectionsignal and a second detection signal according to the first square wavesignal and the second square wave signal. A multiplexor electricallycouples to the two detection circuits for detecting the duration of thesignal according to the first detection signal and the second detectionsignal.

[0019] In accordance with one aspect of the present invention, thesignal is a CAS tone.

[0020] In accordance with one aspect of the present invention, the firstfrequency is 2130 HZ.

[0021] In accordance with one aspect of the present invention, thesecond square wave signal appears earlier than the first square wavesignal according to the signal.

[0022] In accordance with one aspect of the present invention, thesecond square wave signal leads the square wave signal.

[0023] In accordance with one aspect of the present invention, thecircuit further comprises another band pass filter for handling a secondfrequency of the signal.

[0024] In accordance with one aspect of the present invention, thesecond frequency is 2750 HZ.

[0025] In accordance with one aspect of the present invention, thecircuit further comprises two hysteresis comparators electricallycoupled to the another band pass filter for forming a third square wavesignal and a fourth square wave signal.

[0026] The present invention may best be understood through thefollowing description with reference to the accompanying drawings, inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Table 1 is a conventional Bellcore specification table;

[0028]FIG. 1 is a conventional Vi-Vo curve of the hysteresiscomparators;

[0029]FIG. 2 is a conventional CAS tone detection circuit block diagram;

[0030]FIG. 3 is the dual hysteresis comparators signal detection circuitblock diagram according to the present invention;

[0031]FIG. 4 is the signal waveforms diagram for the input and theoutput of the dual hysteresis signal detection circuit filter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0032] Please refer to FIG. 3 showing the dual hysteresis comparatorssignal detection circuit block diagram. In order to solve the problemcaused by damping effect and not to impact the detection sensitivity,two hysteresis comparators (the first hysteresis comparator 321 and thethird hysteresis comparator 323) are added to the original circuithaving second hysteresis comparator 322, fourth hysteresis comparator324 and detection circuit 332. Comparing to the conventional circuit,the hysteresis value of the first hysteresis comparator 321 and thethird hysteresis comparator 323 is larger than the original. When thedamping amplitude is lower than the larger hysteresis value, the signalof the first hysteresis comparator 321 and the third hysteresiscomparator 323 will not change, and therefore the detection circuit willdetect the CAS tone to be ended. Hereby, the CAS tone length (theduration time) detected will be near the actual length. Wherein, thefirst hysteresis comparator output is the first square wave signal S1,the second hysteresis comparator output is the second square wave signalS2, the third hysteresis comparator output is the third square wavesignal S3, the fourth hysteresis comparator output is the four squarewave signal S4, the 2130 HZ band pass filter 311 output is the signal S5having the first frequency 2130 HZ, and the 2750 HZ band pass filter 312output is the signal S6 having second frequency 2750 HZ.

[0033]FIG. 4 shows the relation between the band pass filter and thehysteresis comparators' output. In the conventional design, the outputCAS_ALGO is outputted by the detection circuit 25 (refer to FIG. 2). Inour new architecture, the CAS_ALGO is outputted by the multiplexor 34that can select the first detection circuit 331 with output ALGO_(H) orthe second detection circuit 332 with output ALGO_(L). When the CAS toneis inputted, the second detection circuit 332 will detect the appearanceof the CAS tone earlier than the first detection circuit 331. While whenthe CAS tone is ended the first detection circuit 331 will detect thedisappearance of the CAS tone earlier than the second detection circuit332. The relations of CAS_ALGO, ALGO_(H) and ALGO_(L) is described next.

[0034] Case 1

[0035] The CAS tone amplitude is more than the upper hysteresis(V_(Hth+) to V_(Hth−)), so the first detection circuit 331 and thesecond detection circuit 332 will calculate out the ALGO_(H) andALGO_(L) respectively. The beginning of CAS_ALGO is dominated byALGO_(L) and the ending of CAS_ALGO is dominated by ALGO_(H).

[0036] Case 2

[0037] The CAS tone amplitude is between the upper hysteresis and thelower hysteresis (V_(Lth+) to V_(Lth−)) . Because it is less than theupper hysteresis, the first hysteresis comparator 321 and the thirdhysteresis comparator 323 output no signal. The first detection circuit331 is iddle and ALGO_(H) is no signal. Certainly, the CAS_ALGO isdominated by ALGO_(L).

[0038] Generally speaking, the larger the input signal amplitude is, themore obvious the damping effect caused by filter will be, and the morelonger the damping time will be also. Anyway, the damping signal powerwill decreased more and more. By way of upper hysteresis, most of thedamping signal will be cut off. Therefore, the signal detected will notbe impacted by the damping effect, and it will not become longer, so thereal CAS tone will be acknowledged.

[0039] The advance of the present invention is to detect the CAS tonesignal exactly. Dual hysteresis comparators is used, and the circuitdesign will be flexible, and eventually it is easy to meet therequirements. The circuit design will be easy to implement by way ofadding some modules to the orignal circuit, while the CAS tone detectionwill be improved. In speaking to the signal required to detect theduration time (ex: DTMF), the dual hysteresis detection can improve thesignal detection result with the least midification on the circuit.

[0040] While the invention has been described in terms of what arepresently considered to be the most practical and preferred embodiments,it is to be understood that the invention need not be limited to thedisclosed embodiment. On the contrary, it is intended to cover variousmodifications and similar arrangements included within the spirit andscope of the appended claims which are to be accorded with the broadestinterpretation so as to encompass all such modifications and similarstructures.

What is claimed is:
 1. A circuit for detecting a duration of a signalwith two hysteresis comparators, comprising: a first band pass filterfor inputting said signal and outputting said signal having a firstfrequency; a first hysteresis comparator electrically coupled to saidfirst band pass filter for comparing with said first frequency of saidsignal to output a first square wave signal according a first highvoltage and a first low voltage; a second hysteresis comparatorelectrically coupled to said first band pass filter for comparing withsaid first frequency of said signal to output a second square wavesignal according to a second high voltage and a second low voltage; afirst detection circuit electrically coupled to said first hysteresiscomparator for detecting a part of said signal having said firstfrequency that is higher than said first high voltage and lower thansaid first low voltage to output a first detection signal according tosaid first square wave signal; and a second detection circuitelectrically coupled to said second hysteresis comparator for detectinga part of said signal having said first frequency that is higher thansaid second high voltage and lower than said second low voltage tooutput a second detection signal according to said second square wavesignal; said duration of said signal being detected according to saidfirst detection signal and said second detection signal.
 2. A circuitaccording to claim 1 wherein said signal is a CAS tone.
 3. A circuitaccording to claim 1 wherein said first frequency is 2130 HZ.
 4. Acircuit according to claim 1 wherein said first high voltage is higherthan said second high voltage.
 5. A circuit according to claim 1 whereinsaid first low voltage is lower than said second low voltage.
 6. Acircuit according to claim 1 wherein said second square wave signalappears earlier than said first square wave signal according to saidsignal.
 7. A circuit according to claim 1 wherein said second squarewave signal leads said square wave signal.
 8. A circuit according toclaim 1 wherein said circuit further comprises: a second band passfilter for inputting said signal and outputting said signal having asecond frequency; a third hysteresis comparator electrically coupled tosaid second band pass filter for comparing with second frequency of saidsignal to output a third square wave signal according to said first highvoltage and said first low voltave; and a fourth hysteresis comparatorelectrically coupled to said second band pass filter for comparing withsecond frequency of said signal to output a fourth square wave signalaccording to said second high voltage and said second low voltage.
 9. Acircuit according to claim 8 wherein said second frequency is 2750 HZ.10. A circuit according to claim 1 wherein said circuit furthercomprises a multiplexor electrically coupled to said first detectioncircuit and said second detection circuit for detecting said duration ofsaid signal according to said first detection signal and said seconddetection signal.
 11. A circuit for detecting a duration of a signalwith two hysteresis comparators comprising: a band pass filter forhandling said signal having a first frequency to output said signalhaving said first frequency; two hysteresis comparators electricallycoupled to said band pass filter for handling said signal having saidfirst frequency and forming a first square wave signal and a secondsquare wave signal; two detection circuits electrically coupled to twohysteresis comparators for forming a first detection signal and a seconddetection signal according to said first square wave signal and saidsecond square wave signal; and a multiplexor electrically coupled tosaid two detection circuits for detecting said duration of said signalaccording to said first detection signal and said second detectionsignal.
 12. A circuit according to claim 11 wherein said signal is a CAStone.
 13. A circuit according to claim 11 wherein said first frequencyis 2130 HZ.
 14. A circuit according to claim 11 wherein said secondsquare wave signal appears earlier than said first square wave signalaccording to said signal.
 15. A circuit according to claim 11 whereinsaid second square wave signal leads said square wave signal.
 16. Acircuit according to claim 11 where said circuit further comprisesanother band pass filter for handling a second frequency of said signal.17. A circuit according to claim 16 wherein said second frequency is2750 HZ.
 18. A circuit according to claim 16 wherein said circuitfurther comprises two hysteresis comparators electrically coupled tosaid another band pass filter for forming a third square wave signal anda fourth square wave signal.